Benzaldehyde is an important starting material in various chemical synthesis, including those relating to the synthesis of scents and flavors. In these applications the benzaldehyde is often required to have a high degree of purity, but unfortunately crude benzaldehyde, and especially benzaldehyde prepared by the oxidation of toluene with a gas containing molecular oxygen, will contain certain impurities that are very difficult to remove. One very significant problem presented by these impurities is that it is particularly difficult to obtain a product from such crude benzaldehyde that will satisfy the requirements of olfactory specifications. Furthermore, the presence of such impurities also causes a quite rapid discoloration of the benzaldehyde upon storage. Such discoloration will occur even at very low concentrations of the impurities, such as a few p.p.m. by weight.
One suggested solution which appears in Japanese Patent Publication 24.467/74 is to purify crude benzaldehyde by treating it with an aqueous solution of sodium hydroxide. However, this method of purification does not give satisfactory results. Benzaldehyde treated in this manner is still found to discolor quite rapidly.